The present invention is generally directed to a method and apparatus for drilling, and more specifically, to a method and apparatus for drilling that uses supercritical carbon dioxide (SCxe2x80x94CO2) as a drilling fluid.
After an oil or gas well has been successfully drilled and completed, it is necessary over the productive lifetime of the well to perform maintenance within the well borehole. This maintenance often includes de-scaling operations, or reworking operations to increase production in older wells. It is quite advantageous to be able to insert equipment into a borehole to perform such maintenance without removing the surface production equipment at the well head. Coiled tubing (CT) has been employed in the past for carrying out maintenance procedures that do not require drilling and can be inserted into borehole through the surface production equipment without removing the equipment.
More recently, CT has been used in conjunction with downhole motors for drilling operations as well as other types of maintenance. Approximately 600 wells were drilled with CT rigs in 1997. In particular, CT drilling has become an accepted practice for creating lateral extensions from existing oil and gas wells, which are useful for increasing production levels in the wells.
While CT drilling equipment can be introduced into a borehole through existing surface production equipment and eliminates the labor and time consuming steps of assembling and disassembling a traditional multi-jointed drillstring, CT drilling has a limited ability to penetrate rock formations. This limitation exists because CT drillstrings have significantly lower thrust and torque capacities than do traditional jointed drillstrings.
The lower thrust is a result of limitations on the weight available to a CT drillstring. A CT drillstring has a maximum weight capacity that is a function of the steel from which conventional CT is fabricated. Increasing the diameter of a CT can yield an increased weight capacity; however, the diameter can be increased only up to the point at which the tube is so large as to be difficult to coil, or so large as to be unable to pass through the surface equipment at the well head. The torque capacity of a CT drillstring is also limited by the tubing diameter.
These thrust and torque limitations consequently limit the rate of penetration of bits at the end of CT drillstrings. It is known that the torque and thrust required for drilling under specific conditions can be greatly reduced by providing high pressure fluid jets at the drill head. Unfortunately, the useful service life of conventional steel CT is inversely proportional to the operating pressure. Conventional CT drilling operations are carried out at operating pressures of under 35 MPa (5,000 psi), to assure the service life of the equipment is sufficient to justify the equipment""s initial capital expense. Such pressures are not sufficient for water jet erosion of rock, which typically requires pressures of at least 15,000 psi. While CT drilling systems can be used with pressures up to 15,000 psi, operation of a CT drillstring at such pressures drastically reduces the service life of the CT drillstring, making operations under such pressure conditions impractical. Alternative CT materials such as titanium and composite may be used to increase pressure capacity, but these materials are considerably more expensive and are not in common use.
Nevertheless, several CT-based drilling systems have been described in the prior art. Despite the relatively poor penetration rate available with CT drillstrings, the advantages noted above weigh heavily in favor of using such equipment.
A CT system useful for drilling a lateral drainage well from within an encased borehole is described in U.S. Pat. No. 5,413,184. A ball cutter is coupled to the tubing and lowered into the borehole. The ball cutter cuts through the borehole casing, and a short distance into the strata beyond the casing. The ball cutter and tubing are wound back to the surface, and the ball cutter is replaced with a nozzle blaster. The nozzle blaster is lowered into the borehole and moved into the opening created by the ball cutter. Fluid is then pumped through the nozzle blaster to cut through the stratum. As noted above, the range of pressures available with the use of CT drillstrings do not provide a generally satisfactory penetration rate for such equipment.
Another CT-based system used to drill a lateral drainage well from within an encased borehole is described in U.S. Pat. No. 5,944,123. A rotatable drill head including at least one fluid port is coupled to a distal end of the tubing, which is lowered into the borehole. The rotatable drill head includes at least one contact member that is adapted to maintain a constant distance between the drill head and the substrate face. Modulation of the rotation of the drill head enables drilling of an off-axis channel, which enables the resulting borehole to have a radius of curvature much smaller than can be achieved with traditional drillstrings. Again, the range of pressures available with the use of CT drillstrings does not provide a generally satisfactory penetration rate, even though a borehole with a desirable radius of curvature can be obtained.
It would therefore be desirable to provide a method by which the penetration rate of CT drillstrings can be increased, thus enabling the benefits of CT drilling operations to be realized without sacrificing performance with respect to the penetration rate of the drill head. It would further be desirable to provide a system that implements this method. The prior art does not disclose or suggest such a method or apparatus.
In accord with the present invention, a method is defined for using either a supercritical fluid or a dense gas to increase an efficiency of a drilling operation performed with a drilling apparatus in a substrate. The method includes the step of providing a material that exists in either a supercritical phase state or a dense gas phase state at the temperature and pressure present in the substrate where the drilling operation occurs. A fluid stream of the material is ejected within a well, and at least a portion of the material in the fluid stream exists in either the supercritical phase state or the dense gas phase state, increasing the efficiency of the drilling operation by providing cooling to the drilling apparatus, removing debris generated by the drilling operation, and/or eroding the substrate.
In one embodiment, the drilling apparatus includes a member adapted to apply at least one of a mechanical cutting action, a mechanical grinding action, and a mechanical shearing action to the substrate. The member is positioned in contact with the substrate.
In another embodiment, a fluid jet is included in the drilling apparatus, which is positioned adjacent to the substrate, such that the fluid jet is directed toward the substrate.
The pressure is preferably controlled at the drilling site to ensure that at least the portion of the material exists in either the supercritical phase state or the dense gas phase state at the drilling site. The material includes either carbon dioxide, methane, natural gas, or a mixture of those materials. The drilling operation includes removing scale deposits from a surface of the substrate, forming an opening in the substrate, or enlarging an opening in the substrate.
When the substrate includes a plurality of pores, the material is caused to penetrate into the pores, which may be formed in a material such as rock or concrete. In one application, the substrate is a well, and the material is used for recovering petroleum, natural gas, or other resources from a geological formation. The drill apparatus then preferably includes a coiled tube adapted to fit within the well. The coiled tube preferably includes a downhole motor and a drill head driven by the downhole motor. The downhole motor can be a turbine motor, a jet rotor, a progressive cavity displacement motor, or a vane motor. The drill head can be a jet erosion bit, a mechanical bit, or a jet assisted mechanical bit.
In one form of the present invention, the downhole motor is energized by causing the material to flow through the coiled tube and the downhole motor. In another form of the invention, a second fluid is provided, and the downhole motor is energized by causing the second material to flow through the coiled tube and the downhole motor.
The pressure at the drill site can be controlled to ensure the material exists as a supercritical fluid or dense gas at the drill site. For example, if the existing well includes a surface choke manifold, the surface choke manifold can be used to control the pressure at the drill site. Alternately or in combination with the surface choke manifold, the well can be capped with drilling mud to control the pressure at the drill site. Preferably, the pressure is caused to exceed 5 MPa, and even more preferably, to exceed 7.4 MPa, when the material is carbon dioxide.
A recovery vessel can be used to collect and recover the material after it has been used to increase the efficiency of the drilling operation, so that the material can be reused.
Another aspect of the present invention is directed to apparatus for using either a supercritical fluid or dense gases in drilling operations. The apparatus includes elements that carry out functions generally consistent with the steps of the method described above.